1. Field of the Invention
The present invention relates to a color ink jet recording method and its apparatus for recording a clear and dense color image by discharging color inks from a recording head. In particular, the present invention concerns a color ink jet recording method and its apparatus using color inks of yellow (Y), magenta (M), cyan (C), green (G), red (R), blue (B) and the like and a black (Bk) ink.
2. Related Art
The ink jet recording method in which characters, figures and the like are recorded by discharging micro-droplets of ink from discharge ports, has advantages in reducing the generation of noise, in lowering the running cost, in reducing the size of the apparatus, and in achieving the coloring. The method, therefore, has been extensively used for printers, copying machines, facsimile and the like.
In a prior art ink jet recording method, a sheet of special paper (coated paper) having an ink absorbing layer has been necessary to obtain a color image being high in color development with little bleed. Recently, a method has been practically used having a suitability for printing on sheets of commercial available xe2x80x9cplain paperxe2x80x9d such as high-grade/medium-grade paper and common PPC paper used for printers and copying machines in large amounts. However, in the case of color recording on a sheet of plain paper, the sufficient printing quality cannot be obtained yet. In this case, the largest problem lies in that the prevention of bleed of ink between color inks used is incompatible with the insurance of the quality of black recording, particularly, the quality of black character recording.
In recent years, to form a color image with little bleed on plain paper by an ink jet recording method, a quick drying ink having a high permeating rate against plain paper has been developed. In the color ink jet recording method, color recording is performed using inks of three colors, cyan, magenta and yellow, and further, using inks of three colors, cyan, magenta, yellow, and a black ink. In the case of using the above-described inks excellent in permeability for color ink jet recording, the recorded image has a high quality with little bleed between colors but is low in density as a whole, and further in the surroundings of the recording image area of each color, the ink is slightly bled along fibers of paper (feathering), resulting in poor sharpness.
The feathering is relatively inconspicuous in a color image area; however, it is conspicuous in a black image area, thus deteriorating the recording quality. In particular, for a black character, it is poor in sharpness and unclear, resulting in the poor recording quality.
Accordingly, to obtain a high quality black color recording less in feathering and higher in density, a black ink used for forming a black image must be of a type having a relatively low permeating rate against plain paper.
For this reason, there has been proposed an ink jet recording apparatus and its method using color inks each having a high permeating rate against paper and a black ink having a low permeating rate against paper. However, in the case of using the inks each having a high permeating rate and the ink having a low permeating rate, at the adjacent boundary portion between a black recording image area and each color recording image area, irregular blur is generated, thus significantly reducing the image quality.
The prevention of irregular blur between black and each color is incompatible with the reduction in feathering of black, and therefore, as the subject of color recording, it has been required to simultaneously achieve the prevention of bleed and the reduction in feathering of black.
Unexamined Japanese Patent HEI 3-146355 discloses a method, wherein a recording area along the boundary area between black and each color is not recorded; however, this method is disadvantageous in that the recorded data is changed.
Another method of preventing bleed of ink in a boundary area between black and each color has been known, wherein a black area along the boundary area between black and each color is formed by overlapping of color inks. In this specification, the black color thus formed is called Process Color Bk (hereinafter, abbreviated as xe2x80x9cPCBkxe2x80x9d).
In the method of forming the black area of PCBk along the boundary area between black and each chromatic color, it is easiest to replace all black data with PCBk data of Y, M and C. Since the PCBk formed of only inks of Y, M and C, each of which is high in permeability, it is not bled at the boundary portion between black and each color. However, the PCBk is very different from the black color formed of the Bk ink in terms of hue, and thereby it is fringed by the boundary portion. For this reason, to adjust the hue of the PCBk, an attempt has been made to added the Bk ink in the black area formed of PCBk to some extent.
However, when an ink having a low permeability is adjacent to an ink having a high permeability, a blurry white image (in this specification, referred to as xe2x80x9cwhite hazexe2x80x9d) is generated at the adjacent boundary portion, as shown in FIG. 7, which greatly exerts adverse effect on the image quality.
The xe2x80x9cwhite hazexe2x80x9d is considered to be generated by the following mechanism. The mechanism will be described with reference to FIGS. 8(a) to 8(c).
An ink having a low permeability is low in wettability against paper, and is generally high in surface tension. On the contrary, an ink having a high permeability is high in wettability against paper and is low in surface tension, and in general, it is added with a surface-active agent for improving the wettablity against paper. FIG. 8 shows an example wherein black ink has a low permeability and color ink has a high permeability. When an area (color ink area) recorded by the ink added with the surface-active agent for improving wettability is adjacent to an area (black ink area) recorded by the ink with no surface-active agent, as shown in FIG. 8(a), the surface tension of ink with no surface active agent at the adjacent boundary is lowered by the action of the surface-active agent, and further the ink with no surface-active agent is applied with internal cohesive force because of its high surface tension and tends to be moved in such a manner as to be formed in a spherical shape. Consequently, as shown in FIG. 8(b), the surface-active agent is spread even along the area recorded by the ink with no surface-active agent, and accordingly, when being fixed in such a state, an area with less dye is generated as shown in FIG. 8(c) and is recognized as the xe2x80x9cwhite hazexe2x80x9d.
As a result, although the bleed of ink at the boundary area between different colors can be prevented, the image is observed as being poor by the generation of the above-described xe2x80x9cwhite hazexe2x80x9d.
One example of a combination of ink compositions used in the case where the xe2x80x9cwhite hazexe2x80x9d is generated, is shown as follows.
Each color ink (C, M, Y) has a low surface tension of about 30 dyn/cm, and a high blotting factor against plain paper (diameter of a dot formed on a recording paper sheet/diameter of a discharged ink droplet) of nearly from 2.5 to 3.5, and accordingly, it is easier to be spread and slightly lowered in sharpness and to be permeated in paper. On the other hand, the Bk ink has a high surface tension of about 50 dyn/cm, and a low blotting factor against plain paper of nearly from 1.7 to 2.0, and accordingly, it is difficult to be spread and to be permeated in paper.
Accordingly, an object of the present invention is to solve the above-described problems, and to provide a color ink jet recording method and an apparatus therefor, wherein even when an image is recorded such that a black ink is adjacent to each color ink using an ink jet recording apparatus having the color inks each having a high permeability and the black ink having a poor permeability, an excellent color image recording with no xe2x80x9cwhile hazexe2x80x9d can be compatible with a black character image recording with less feathering due to prevention of bleed of ink between black and each color.
The above object can be achieved, according to a first aspect of the present invention, by provision of a color ink jet recording method, comprising the step of recording a desired color image on a recording medium on the basis of recording data using a black ink and a plurality of color inks each being different from the black ink in permeating characteristics against the recording medium, wherein the surface tension is 40 dyn/cm or less for each of the black ink and the color inks; and the blotting factor against plain paper is 2.5 or less for the black ink, and it is in the range of from 2.5 to 3.5 for each of the color inks.
According to a second aspect of the present invention, there is provided a color ink jet recording method, comprising the step of recording a desired color image on a recording medium on the basis of recording data using a black ink and a plurality of color inks each being different from the black ink in permeating characteristics against the recording medium, wherein the surface tension is 40 dyn/cm or less for each of the black ink and the color inks; and the permeating rate against plain paper is one second or more for the black ink, and it is less than one second for each of the color inks.